Rechargeable energy storage cells are used in a variety of applications including gas operated motor vehicles and electric vehicles. Of the applications, electric vehicles such as golf carts, fork trucks, marine applications, floor-sweeper scrubber and recreational vehicles and the like are the most demanding in terms of charge/discharge cycles. Energy storage cells for such electric vehicles are commonly known as "deep cycle" batteries which provide relatively steady power over extended periods of time between charging and which operate in a deep cycling mode of severe discharging as well as daily recharging cycles. Deep cycle energy cells are desirably recharged with little or no supervision. Accordingly, the cells must be capable of multiple charge/discharge cycles without significantly degrading of the power output properties of the cells. Conventional rechargeable energy storage cells have properties which do not lend themselves to the rigorous duty cycles of the deep cycle batteries.
Most energy storage cells of the nature described above contain positive plates or grids made of lead-antimony alloys which are easier to cast and produce stronger electrodes. The antimony also improves the corrosion resistance of the positive plate to acid attack and increases the ability of the battery to recovery from deep discharge cycles. However, during overcharge and other forces in the cells, the antimony tends to oxidize and dissolve out of the lead alloy plates. Once dissolved in the electrolytic solution, the antimony compound travels through the separator between the positive and negative plates and deposits or plates out on the surface of the negative plate. This layer of antimony oxide tends to reduce the active surface area of the negative plate and thereby reduce the capacity and voltage behavior of the battery. Overcharge of the energy cell is more likely to occur because the cell's charge voltage has decreased which further exacerbates the deposition of antimony oxide and lowering of charge voltage which hastens the deterioration of the life of the energy cell. Antimony also tends to cause a lowering of the hydrogen overvoltage which can lead to the formation of undesirable quantities of hydrogen gas in the energy cell. In order to inhibit the antimony transfer from the positive to the negative plates, rechargeable deep cycle energy cells preferably include separators which exhibit the ability to retard antimony transfer to the negative plates and which give high end of charge voltage as well as reduce or prevent dendrite growth in the cells.
There are several types of separators which are commercially used in rechargeable energy cells. The separators differ by the material composition and include rubber separators, polymeric separators such as polyethylene separators, polyvinyl chloride (PVC) separators, phenolic resorcinol separators, fiberglass separators and resin impregnated cellulosic paper separators. The separators are further classified as microporous separators and macroporous separators. The microporous separators include separators made of natural rubber, polyethylene, phenolic resin, PVC and polymeric membranes. Macroporous separators include separators made of glass fiber mats, sintered PVC and resin-impregnated cellulosic papers. Of the foregoing, microporous, natural rubber separators typically exhibit the best electrochemical performance characteristics which enhance the overall performance of the energy cell.
Because of the inherent limitations of rubber separators, attempts have been made to use more flexible polyolefin separators. U.S. Pat. No. 3,351,495 to Larsen et al. describes a conventional microporous polyolefin separator which contains a microporous sheet of polyolefin having a molecular weight of at least 3,000,000 and which contains 7 to 92 volume percent filler. The filler is said to be selected from carbon black, coal dust, graphite, metal oxides and hydroxides, metal carbonates, minerals, synthetic and natural zeolites, portland cement, precipitated metal silicates, alumina silica gels, wood flour, wood fibers, bark products, glass particles, and salts. The preferred filler is said to be finely divided synthetic, precipitated silica.
U.S. Pat. No. 4,237,083 to Young et al. describes a process for making a microporous sheet by forming a blend of polyolefin, silica and a water insoluble plasticizer, forming a sheet from the blend and contacting the sheet with water for a time sufficient to render the sheet microporous. The resulting sheet material is said to have good electrical resistance characteristics.
There are two primary functional aspects of separators used for energy cells, one is physical and the other electrochemical. The important physical characteristics include high porosity, small mean pore diameter, oxidation resistance, puncture resistance, thermal dimensional stability and low levels of harmful chemical contaminants. Electrochemical characteristics of importance include favorable voltage characteristics, retardation of antimony transfer, acceptable Tafel behavior, and prevention of dendrite growth. The Tafel behavior of an energy storage cell is a determination of the hydrogen and oxygen over-potential shifts in the cell electrolyte compared to pure acid solutions. The electrochemical compatibility test enables a prediction of the long term effect of chemical residues leached into the electrolyte from the separators.
Despite the advances made in the art with respect to improved separators, there continues to be a need for separators for energy storage cells which exhibit improved physical and electrochemical properties over conventional polyethylene separators.